Optoelectronic Properties of TiS2: A Never Ended Story Tackled by Density Functional Theory and Many-Body Methods

Herein is reported a thorough computational investigation on the bulk TiS2 material with the CdI2 structure type and the ideal 1:2 Ti:S stoichiometry. Computations were performed using some of the most refined models, e.g., a hybrid functional together with dispersion effects (Grimme's), the GW ansatz, and the Bethe-Salpether equation for the optical properties. We showed that switching from Perdew-Berke-Enzerhof (PBE) to PBE0 leads to a gap opening. Moreover, our results demonstrate unambiguously that van der Waals interactions must be properly treated with dispersion effects in order to retrieve the experimental crystal structure and the appropriate c/ a ratio. Indeed, the calculations prove that when one uses a highly accurate computational protocol, the bulk hexagonal TiS2 is a semiconductor with a small gap, whereas using the generalized gradient approximation (GGA) PBE functional leads to a semimetal. Furthermore, the band structure is significantly modified when dispersion parameters are taken into account. Pressure effects were also investigated, and they fully describe the previously simulated electronic transition behavior of the material, e.g., TiS2 becomes semimetallic under strain.